Clean up the keepalive slots when network disconnect

In general, keepalive slots are released after result of
stopping has returned. However, for network disconnect case,
the service side cannot communicate with network agent since
the async channel is broken.

Clean up keepalive slots right after stop in this case.

Bug: 132341736
Test: 1. atest com.android.server.ConnectivityServiceTest \
         #testNattSocketKeepalives --generate-new-metrics 100
      2. atest FrameworksNetTests --generate-new-metrics 10

Change-Id: Id3e4e159713c0ed7e03f45169e87b73ae6408e4f
(cherry picked from commit a5f6bd16)
Merged-In: Id3e4e159713c0ed7e03f45169e87b73ae6408e4f
Merged-In: Icb5a1b5bb10617aa5a7b35db6cf48db3dc53b7fd